The Aztec origins of four obsidian artefacts held by the British Museum, including a round mirror associated with the Renaissance polymath and confidant of Elizabeth I John Dee (1527-1608/1609), have been confirmed by researchers from the University of Manchester, the Sobolev Institute of Geology and Mineralogy, and the University of Missouri. This team, led by Professor Stuart Campbell (Manchester), used portable X-ray fluorescence (pXRF) to trace the provenance of the items, all of which lacked secure archaeological contexts having been transported to Europe after the Spanish invasion of the Americas in the 16th century.
As discussed in previous ‘Science Notes’ (see CA 367 and 379), pXRF is used to reveal the elemental make-up of archaeological samples. X-ray beams are emitted from the pXRF instrument, hitting the sample’s atoms, which in turn emit X-rays with energies specific to particular elements. The machine then identifies each element and its quantity, and these results can be used to determine provenance of a sample by comparison with the elemental composition of known reference samples. The method is quick and non-destructive, and, in this case, the portable nature of the instrument allowed the researchers to sidestep the difficult process of arranging for delicate objects to be removed from their museum context.
The artefacts examined by the team included ‘the John Dee mirror’, two similar circular mirrors, and a rectangular slab. All of the objects were made of obsidian, which is ‘basically a naturally occurring glass that is formed by volcanic eruptions,’ said Stuart. In terms of pXRF analysis, obsidian is particularly easy to work with, he added: ‘there aren’t so many sources around, they’re – relatively speaking – fairly homogeneous in composition, and, unlike something like pottery, where you’re adding stuff in, obsidian doesn’t really change in composition when it is used to manufacture objects.’
The researchers took three readings in two locations on each mirror, and compared their results to elemental data derived from Mexican rock samples from Pachuca, Ucareo, and Zaragoza. The geochemical compositions of ‘the John Dee mirror’ and another of the circular items were found to match obsidian from Pachuca, while the geochemical fingerprints of the third round mirror and the rectangular slab corresponded to data relating to obsidian from Ucareo. Although pXRF cannot help date the items, circular mirrors of the type analysed are known from illustrations in codices created by indigenous artists who lived towards the end of the Aztec empire.
The findings were recently published in Antiquity, where the research team explains that, for the Aztecs, obsidian had religious, mythological, and ritual meanings and was thought to offer protection against evil spirits. The material was particularly associated with the god Tezcatlipoca, whose obsidian mirrors were linked to ‘revelation, premonition, and power.’
‘The John Dee mirror’ seems to have subsequently been used in a magical context in Europe, as part of Dee’s well- attested interest in the occult and divination. John Dee is known to have employed the medium Edward Kelley to communicate with angels using mirrors, and left many of his possessions to John Pontois, including a stone disc thought to be his obsidian mirror. ‘I don’t think I’ve ever worked with an object that’s such a striking example of how an object can have evolving meanings at particular points during its journey through its possession by different people,’ said Stuart, noting that Dee’s Aztec mirror is ‘still acquiring new meanings and new associations on display in the British Museum.’